This invention relates to improvements in systems for controlling the movement of a train along a railroad track and, more particularly, to a train control system which integrates dynamic and fixed data concerning the stretch of track over which the train is travelling and conditions existing on the track ahead, and which determines train control instructions from such data and has the capability of enforcing any restrictive instructions that are not obeyed.
Railroad signalling and train control systems have traditionally been based on the concept of protecting zones of track, called "blocks," by means of some form of signal system that conveys information to the locomotive engineer about the status of one or more blocks in advance of the train. Wayside signal lights located along the track are controlled by electrical logic circuits which use track circuits to detect the presence of a train in any given block, and automatically combine the status of several adjacent blocks to present the proper aspect, or combination of lights, to indicate to the train crew whether the train may proceed at maximum speed, should reduce speed due to more restrictive conditions ahead, or should be brought to a stop. The distance required to slow or stop a moving train is sufficiently long that information must be conveyed to the train at least one full block in advance of where the reduced speed or stop is required.
An alternative approach which is used on portions of some railroad systems is referred to as cab signalling and may be used with or without wayside signal lights. In cab signalling the same logic that determines block status for display on the wayside signals is also used to generate one of several forms of encoded electrical current in the rails, such that block status is represented by the selection of the code rate used. Equipment on the locomotive detects the coded currents through inductive pickup coils located just above the rail and ahead of the lead wheels, and decodes the information to arrive at a status to be displayed in the engine cab in the form of a pattern of lights similar to those used on wayside signals. The particular pattern of lights displayed is called the "aspect" of the signal. Displaying this information in this manner makes the block status visible to the train crew continuously, not just while approaching a wayside signal, and also permits any change in block status to be displayed immediately as it happens rather than at the next wayside signal which may be far ahead and out of sight at the time of the change in status.
Most cab signal systems include some form of automatic train control (ATC) feature which uses one or more methods to assure that the train crew is alert and responding to any changes in cab signal aspects. Some of these systems only require acknowledgement of the change, while others require application of brakes within a minimum time interval as assurance that a more restrictive condition is recognized by the crew. Some more refined ATC systems also have a target speed associated with certain of the aspects, and enforce the reduction in speed until the target speed is reached. In any of these enforcements, the consequence of an engineer failing to respond in the proper manner is an automatic penalty brake application which generally forces the train to come to a full stop before the engineer is able to regain manual control of the brakes and begin moving again.
Some high density passenger railroads involved in commuter or transit operations use the cab signal coded information exclusively to display an authorized speed to the engineer, rather than a pattern of lights conveying block status. The number of speeds that may be displayed is limited to the number of codes available in the wayside equipment, which is typically from three to six. This essentially prevents the use of these codes for conveying speed limits for any purpose other than nominal values resulting from changes in signal aspects. However, a railroad line typically has a number of areas, such as curves and bridges, where fixed civil speed restrictions are imposed for safety, but automatic indication and enforcement of such speed restrictions is outside the scope of a conventional cab signal system.
Furthermore, except on the high density passenger lines, a cab signal system has also not been able to convey enough information to indicate when an absolute stop is required, due to a potential conflicting route situation, as opposed to a "restricted speed" type of movement in which one train may be following another and be required to operate on visual rules at a speed slow enough to be able to stop short of another train, obstruction or open track switch. Inability to make this distinction of course prevents the conventional system from enforcing a complete stop ("positive" stop) at the proper location. Since these stops cannot be enforced, there are accidents occasionally, even in cab signal territory, caused by a train crew inadvertently running past a stop signal and into the path of another train.
Additionally, since train operations often span several rail lines having different cab signal systems, or none at all, there is a need for a reliable automatic means for changing the operational mode of the on-board train control equipment.